Heat treatment of aluminum base alloys containing tin



3,1?6fi57 IEAT 'lllEA'ifl/KENT 3F ALUlldlNUll/l BASE ALLGYS EGNTAlNlNG TlN Michael J. Pryor and Douglas S. Keir, Hanzden, and Philip R. Sperry, North Haven, llama, assiguors to Olin Mathiesou Chemical Corporation No Drawing. Filed June 19, 1964, Ser. No. 376,545 7 Claims. (Qt. 148-159) This is a continuation-in-part of United States Patent application Serial No. 653166 filed October 3,196l), United States patent application Serial No. 171,114 filed February 5, 196-2, and United States patent application Serial No. 251,924 filed January 14-, 1963.

The present invention relates to a process for treating aluminum base alloys containing tin. More particularly, the present invention relates to a process for treating aluminum-tin containing alloys so as to obtain significant improvements in their physical and/or chemical characteristics.

has been found in accordance with the present invention that when aluminum tin containing alloy is treated in accordance with the specific process conditions of the present invention a significant and surprising improvement in various physical characteristics of the alloy is obtained. For example, and in particular, the process of the present invention attains a surprising and in fact remarkable improvement in anodic efi'iciency. Anodic efficiency is a conventional term and means the ratio of the weight of consumed anode which goes directly into prod cing electric current from Faradays law to the actual total weight of the anode consumed, usually expressed as a percent. Higher efficiency means less anode wastage due to local corrosion and, hence, a lower cost of cathodic protection, longer life for the anode material, less corro sion by-products such as insoluble hydrated oxides and gaseous hydrogen, and more uniform delivery of galvanic current over the useful life of the anode. in addition, when the alloy is used as an electrical conductor there is attained improved properties in aluminous electrical condoctors in combination with increased tensile strength and more favorable A.C.-D.C. ratios. In this regard see copending application Serial No. 313,445 filed October 3, 1963.

it has been found in accordance with the present invention that the foregoing improvements may be obtained by (1) providing an aluminum base alloy containine at least 96% aluminum and from 0.04 to 0.5% tin; (2) heating said alloy to a temperature of from 540 C. to 640 C. for at least 15 minutes, and preferably from 15 iinutes to 24 hours; (3) rapidly cooling said alloy to a temperature of at least 150 C. at a rate of at least 80 C. per hour, thereby retaining the tin in solid solution to the .iaximum degree. In the preferred embodiment the cooling rate is quite rapid and normally it is preferred to bring the temperature down to 150 C. in 15 minutes or less.

' For convenience and simplicity the foregoing process may be termed homogenization treatment.

The equilibrium solid solubility of tin in aluminum is very low, being less than 0.02% at 228 C. and decreasing still further as the temperature is lowered. The maximum solubility of tin in solid aluminum is 0.10% and occurs at 629 C. It was shown in copending application Serial No. 60,166 that the tin is most edective in forming the desired n-type defect structure in the aluminum oxide film at the metal surface if the tin is maintained to the maximum degree in solid solution. Particles of tin which are randomly dispersed and are not in solid solution will make some contribution to the desired defect structure but they necessarily cannot affect the film at more than an extremely small distance from their 3,l%,@5? Patented July 20, 1965 vicinity. Therefore, if the tin is not retained in the amount of its maximum solubility, the next best arrangernent is to have the undissolved tin subdivided into the largest possible number of small particles consisting of more than one tin atom each.

It is well known that the solidification of any alloy having some solid solubility of the solute element in the solvent results in a nonbomogeneous distribution of the solute element known as microsegregation or coring. Accordingly, some parts of the alloy will retain the maximum solid solubility but others will not. Even the most rapid solidification obtainable in practice cannot yield a homogeneous solid solution and, on the other extreme, slow solidification which would allow equilibrium to be attained is not practical. Whereas the degree of homogencity can be altered over a Wide range by the particular casting practice used, the only way to obtain a uniform solid solution content is to hold the alloy at a temperature at which the solubility is relatively high and diffusion rates are sufiiciently great to attain the equilibrium solubility uniformly throughout the alloy.

Even when the maximum solubility is attained at an elevated temperature, it should not be theoretically possible to retain this condition at room temperature if the equilibrium phase diagram shows that solubility decreases with lowering temperature. However, in practice, it is known that solid solutions which are supersaturated at room temperature can be retained if cooling is sufiicient- 1y rapid. Alternatively, some decomposition may occur on cooling but, if the cooling rate is sufiiciently rapid, the particles which precipitate will be extremely fine and well dispersed.

It is recognized that some casting processes, e.g., the direct chill (D11) continuous or semi-continuous process, by virtue of their rapid solidification rate on moderately small section thicknesses and the continuation of rapid cooling to room temperature, produce a structure in which a large portion of the tin is retained in supersaturated solid solution. However, there still remain regions in which th re is less than the maximum amount and also regions where the excess tin is concentrated in particulate form. Therefore, DC. castings may have galvanic characteristics which approach those of homogenized material and could conceivably be used in the as-cast condition with some sacrifice in optimum performance. However, DC. casting of large section thicknesses or other casting methods, e.g., sand and permanent mold, will result in even greater departures from the ideal homogenized structure and will require a separate homogenization heattreatment.

If the aluminum-tin alloy is to be wrought after casting, e.g., by rolling, extruding, or forging, the temperatures at which these operations and any attendant annealings are performed are generally well below that at which solubility is at a maximum and decomposition by precipitation from solid solution will occur. Therefore, a homogenization heat-treatment to restore the maximum solid solubility, followed by cooling to retain the effects of maximum solid solubility, is a necessity for optimum galvanic characteristics on wrought alloy products.

It is important to the understanding of this invention that theoptimum combination of high galvanic current and anodic efiiciency are obtainable when the tin is most uniformly distributed and retained in solid solution to the highest degree, but that, within a broad range, departures from this ideal may still yield satisfactory, though inferior, characteristics. Furthermore, some im provements in either galvanic current or efficiency may be brought about by the addition of other alloying elements as will be apparent hereinafter.

The present invention relates to aluminum base alloys containing at least 96% aluminum and from 0.04 to 0.5%

'tice.

tin. Naturally a wide variety of alloying additions may be readily tolerated and are in fact preferred. In addition to this conventional impurities normally associated with aluminum base alloys may of course be present, for example, silicon in an amount up to 0.10% and iron in an amount up to 0.1%.

Generally, insoluble elements may be added to the alloy, i.e., elements which have less than 0.03% maximum solid solubility in aluminum. The total amount of these insoluble elements should be no greater than 0.5%. These insoluble elements have no significant effect on current output as they do not reduce the solid solubility of tin in aluminum, but they act as second phase particulate cathodes and large amounts ultimately reduce anodic efficiency by promoting local corrosion of the anode.

Soluble elements may also be added to the alloy. The soluble elements may be considered either lattice expanders or lattice contractors, i.e., ternary addition elements which either expand or contract the aluminum lat- Generally lattice expanders stabilize tin in retained solid solution and permit high galvanic currents to be drawn from the alloy. Lattice expanders may be used in an amount from about 0.001 to 8%, with typical lattice 'expanders and amounts thereof which may be used including: magnesium from about 0.001 to 7.0%; zirconium from about 0.001 to 0.3%; bismuth from about 0.001 to 0.5%; indium from about 0.001 to 0.5; and mixtures thereof.

Lattice contractors generally reject tin from solid solution, but small amounts may be tolerated, for example, zinc up to 0.01%; copper up to 0.002%; silicon up to 0.10% and manganese up to 0.05%.

The manner of bringing the alloy to the elevated temperature is not especially critical. It is critical to retain the alloy at a temperature of from 540 C. to 640 C. for at least minutes and preferably from 15 minutes to 24 hours. The preferred treatment temperature is from 600 to 630 C., and optimally 620 C.

After the heat retention step the alloy is critically rapidly cooled to a temperature of at least 150 C. at a rate of at least 80 C. per hour and optimally the alloy is brought down to 150 C. within 15 minutes This is an essential'feature of the present invention. It is this rapid cooling step which forcibly retains the maximum amount of tin in solid solution, on one extreme, for example, water quenching, or at least in combination with substantially the maximum amount of tin in solid solution this step provides a fine dispersion of tin particles as pointed out above and thereby achieves the surprising features of the present invention. The more rapidly the alloy is cooled the more efficient is the retention of tin in solid solution, thus naturally the preference for rapid cooling methods, such as immersion of the alloy in cold water.

In addition to the favorable metallurgical structure which is produced by the homogenization treatment of -the present invention, it has been surprisingly found that the air formed oxide which is formed on the surface of i the alloy during heat treatment is favorable for promoting a fast activation of the anode, which fast activation is highly desirable for some applications, for example, sea water batteries.

The present invention will be more readily apparent from the considertaion of the following illustrative examples.

EXAMPLE I An aluminum base alloy was cast by conventional D.C. casting methods using a 99.85% pure aluminum with as alloying additions pure tin and bismuth. The resultant alloy had a chemical composition when analyzed after casting of 0.12% tin and 0.15% bismuth and the remainder essentially aluminum. Some of the ingots were then given varying homogenization treatments, that is, some of the ingots were heated to temperatures between 575 C. and 620 C. and held there for various times followed by cooling at various rates to room temperature.

All of the ingots, including the as-cast and homogenization heat treated ingots were given various tests to determine their galvanic characteristics.

Specimens were prepared from sections of the ingots by machining. Specifically, the specimens used for deter'mining galvanic properties were milled to 0.197 x 0.197 inch (5 mm. x 5 mm.) in cross section and '3.inches (75 mm.) length. They were chemically cleaned and a 10 sq. cm. area was exposed in a Galvanic Cell Test substantially as described in an article in the journal of the Electrochemical Society, volume 105, No. 11, starting at page 629. All determinations were carried out in 0.1 N sodium chloride solution at 25:0.1 C. The galvanic currents were measured continuously by shorting the cell through a 1 ohm resistance and continuously recording the drop in potential. A second set of similar test specimens was subjected to a modified test arrangement, Impressed Current Test, for which 1 liter of 1.0 N NaCl solution was used and a constant current density of 10 met/cm. was maintained on the specimen, as the anode, for 24 hours, employing a 10 cm. steel cathode. This current density was at least 10 times greater than that in the Galvanic Cell Test and it approximates conditions obtained in a larger scale galvanic test where the cathode area is many times larger than the anode area and where low eificiencies may be obtained if there is a tendency to form spongy corrosion product.

Electrochemical tests of the above two types produce a fairly large scatter. Therefore, individual test results are shown. The number of coulombs flowing in 48 hours in the Galvanic Cell Test is a measure of the ability of the anode to maintain a protective current and depends upon the maximum tin being in solid solution or substantially the maximum tin in solid solution with the remainder very finely dispersed. The overall percent efficiency is the anodic efiiciency. The weight of sponge in the Impressed Current Test refers to the amount of lightly adhering corrosion product containing entrained metallic particles which contributes to low efficiency.

The results are shown in the following table.

TABLE I Galvanic cell test Impressed current test,

10 ma./sq. cm. Homogenlzation, temperature-time-eooling Coulqmbs Overall Total wt. Overall fiow1ng percent of sponge, percent in 48 hrs. etficiency rue/cm. eflicieney As-east by DC casting 1013,1028,1071 53, 53, 52 16. 8, 14. 6, 8. 2 45, 52, 53 620 C., 16 hrs., water quench 934, 900, 1003 53, 50, 55 0, 0, 0 71, 71, 72 1007, 1113, 1082 57, 64, 61 3. 3, 0, 0 61, 72, 76 620 C., 6 hrs., water quench 835, 948, 914 42, 54, 54 3. 5, 1. 1, 1. 1 55, 68, 67 620 C., 16 hrs., air cool 757, 892, 951 56, 57, 64 1. 3, 2. 6, 4. 6 74, 59, 620 C., 16 hrs., furnace cool at 80 C./hr. to 260 0., air cool to room temp 793, 796, 839 62, 68, 68 4.1, 4. 1, 3. 5 66, 71,67 605 C., 10 hrs., air c0ol- 789, 997, 891 59, 57, 55 3. 1, 0.6, 0.6 67, 76, 605 C., 10 hrs., water qu 968, 950 4 4 0, 0, 11. 4 70, 71, 74 595 C., 10 hrs., water quench. 841, 866, 820 48, 50, 51 2. 7, 1. 0, 1. 0 62, 70, 69 575 C., 10 hrs., Water queneh 900, 905 54, 48 0, 0, 0 68, 69, 69

The optimum condition of homogenization in the foregoing table is represented by the heat treatment consisting of 620 C. for 16 hours followed by a water quench. It is observed that the as-cast D.C. casting delivers about the same galvanic current but produces slightly lower efficiency on the average at low current density and markedly lower efficiency with high sponge formation at high current density. Reducing the time of homogenization heating to 6 hours or decreasing the cooling rate by air cooling or furnace cooling effects some sacrifice in either galvanic current anodic eificiency or both. Likewise lowering the temperature to 605 C., 595 C., or 575 C., all of which are below the temperature of maximum solid solubility for tin (620 C.), shows a definite tendency for lowering the galvanic current and efiiciency. Other casting methods in which the soldification rate is lower than that for DC. casting also lower the galvanic current, as shown in co-pending application Serial No. 60,166, FIGURE 1 thereof.

EXAMPLE II An aluminum base alloy having the following composition: 0.30% tin, 0.0028% iron, less than 0.001% each of silicon and copper and the balance aluminum was cast into 5 identical ingots. Subsequent to casting the ingots were given the following treatments in order to get them from the initial 2%" thickness down to a final thickness of 0.010".

Ingot A was hot rolled to 0.25" at 260 C. The ingot was then homogen zed at 620 C. for one hour followed by quenching in water. The ingot was then cold rolled 96% to 0.010" thickness. The ingot was then re-homc-genized at 620 C. for 30 minutes and quenched in water. This ingot was in the dead soft temper and contained the maximum amount of tin in solid solution.

Ingot B was treated in the identical manner as ingot A with the exception that the re-homogenization step was omitted. This ingot was in the 96% cold rolled temper and contained the maximum amount of tin in solid solution.

Ingot C was homogenized for 8 hours at 620 C. and furnace cooled at a rate of 55 C. per hour to 260 C. It was then hot rolled at 260 C. to 0.25 thickness followed by cold rolling 96% to 0.010" thickness. This material was then in the 96% cold rolled temper and the tin was precipitated from solid solution at 260 C. Thus this alloy contained substantially no tin retained in solid solution.

Ingot D was hot rolled to 0.25" thickness at 260 C. followed by homogenizing at 620 C. for one hour and quenching in Water. The ingot was then cold rolled 80% to 0.050" thickness, annealed at 315 C. for one hour, air cooled and finally cold rolled 80% to 0.010" thickness. This alloy was in the 80% cold rolled temper with the tin precipitated from solid solution at 315 C. Thus the alloy contained substantially no tin in solid solution.

Ingot E was treated in a manner identical with ingot D with the exception that the intermediate anneal was replaced by a partial anneal at 120 C. for 3 hours. This alloy was in the 80% cold rolled temper and contained tin in solid solution to the maximum degree since the thermal treatment at 120 C. was too low to cause precipitation of the tin from solid solution.

Test cells were prepared with the alloys prepared above and silver-silver chloride cathodes in the manner of Example II of co-pending application Serial No. 251,024. The test procedure was conducted so that the data obtained was the current density-time characteristics and the cell voltage across a fixed external load resistance. The maximum current density maintained over a significant portion of the total time of operation of the cell indicates the usefulness of the anode material. The results obtained from the test cells are given in the table below, wherein cells A through E utilize the alloys prepared from ingots A through E, respectively.

The foregoing results show conclusively that when the tin is retained in solid solution to the maximum degree a surprising benefit is obtained in anode applications. The current density of cells C and D, using alloys which were given thermal treatments enabling the tin to precipitate from solid solution, was only about 57% of that obtained in the other samples in which the beneficial effects of the homogenizing heat treatment were obtained.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. A process which comprises:

(A) providing an aluminum base alloy consisting essentially of at least 90% aluminum and from 0.04 to 0.5 tin;

(B) heating said alloy to a temperature of from 540 C. to 640 C. for at least 15 minutes; and

(C) rapidly cooling said alloy to a temperature of lower than 150 C. at a minimum rate of C. per hour, thereby retaining the maximum amount of tin in solid solution.

2. A process according to claim 1 wherein said heating is at a temperature of from 540 C. to 640 C. for from 15 minutes to 24 hours.

3. A process according to claim 2 wherein said tempera ture is from 600 to 630 C.

4. A process according to claim 1 wherein said alloy is rapidly cooled to a temperature of lower than 150 C. in less than 15 minutes.

5. A process which comprises:

(A) providing an aluminum base alloy consisting essentially of at least aluminum, from 0.04 to 0.5% tin, up to 0.10% silicon and up to 0.10% iron;

(B) heating said alloy to a temperature of from 540 C. to 640 C. for from 15 minutes to 24 hours; and

(C) rapidly cooling said alloy to a temperature of lower than C. in less than 15 minutes.

6. A process according to claim 5 wherein said alloy contains from 0.001 to 8 percent of at least one soluble element which expands the aluminum lattice.

7. A process according to claim 6 wherein said soluble element is selected from the group consisting of from 0.001 to 7.0% magnesium, from 0.001 to 0.3% zirconium, from 0.001 to 0.5% bismuth, from 0.001 to 0.5 indium, and mixtures thereof.

References Cited by the Examiner UNITED STATES PATENTS 1,629,699 5/27 Guertler et al. 148-159 2,087,992 7/37 Nock 148-159 2,225,925 12/40 Nock 148-32 2,796,456 6/57 Stokes 136-100 2,886,432 5/59 Schmitt et a1 75-138 3,063,832 11/62 Snyder 75-138 FOREIGN PATENTS 636,433 4/50 Great Britain.

HYLAND BIZOT, Primary Examiner. DAVID L. RECK, Examiner. 

1. A PROCESS WHICH COMPRISES: (A) PROVIDING AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF AT LEAST 90% ALUMINUM AND FROM 0.04 TO 0.5% TIN; (B) HEATING SAID ALLOY TO A TEMPERATURE OF FROM 540* C. TO 640*C. FOR AT LEAST 15 MINUTES; AND (C) RAPIDLY COOLING SAID ALLOY TO A TEMPERATURE OF LOWER THAN 150*C. AT A MINIMUM OF 80*C. PER HOUR, THEREBY RETAINING THE MAXIMUM AMOUNT OF TIN IN SOLID SOLUTION. 